Outside-the-ribcage thoracic migration of a spigelian hernia.

Lung and spigelian hernia are both extremely rare diseases, and their combined and simultaneous appearance in the same patient, causing the small bowel to shift from the abdomen towards the thorax external to the ribcage is even more unusual. Here, we report such a case, supported by comprehensive iconography and a detailed discussion of the hypothetical pathogenesis.

On the origin and dynamics of the vasomotion of small arteries.

A system of differential equations describing stationary vasomotion is formulated. It incorporates the ionic transports, cell-membrane potential, muscle contraction of the vessel smooth muscle cells, and the mechanics of a thick-walled cylinder. It is shown that the interaction of Ca2+ and K+ fluxes mediated by voltage-gated and voltage-calcium-gated channels, respectively, brings about periodicity of those transports. This results on a time-periodic cytoplasmic calcium concentration, myosin light chains phosphorylation, and crossbridges formation with the attending muscle stress. The vessel's transmural pressure determines a hoop stress. The resultant hoop, elastic, and muscle stresses determine the rate of change of the vessel's diameter: vasomotion. The model results agree with the experimental observations. The sensitivity of the vasomotion's dependence on parameter values and its significance to experimental protocols are examined. Further, it is hypothesized that the dependence of calcium-channel openings on voltage is shifted by changes on transmural pressure. Thus, Harder's experimental results are reproduced, among them the decreasing of vessel diameter with increasing pressure. Those behaviors are associated with a pattern of change of the singularities of the system of equations describing the model. This suggests a functional relationship on the interactions of Ca2+ and K+ fluxes responsible for the myogenic response; it may not result from a single molecular mechanism. The model is constructed so that additional experimental information can be readily incorporated.

Parameter dependence of myoglobin-facilitated transport of oxygen in the presence of membranes.

Some physicochemical entities involved in the facilitated transport of oxygen along a transport path z1 less than or equal to z less than or equal to zn with membranes impermeable to myoglobin at zi, i = 1,...,n, were identified in an earlier paper [Math. Biosci. 95:209 (1989)]. These entities are the partition between the oxygen and oxymyoglobin flows, the flow transfers taking place near a membrane, and the membrane resistance. Expressions for these entities were obtained that explicitly involve the parameters of the system. In this paper, for the case of prescribed boundary oxygen concentrations x1 and xn, these expressions are incorporated into (i) an explicit representation for the facilitated transport value in terms of the difference, E(x1)-E(xn), between the boundary oxymyoglobin equilibrium values and the sum, gamma, of the membrane resistances, and (ii) a representation for the distribution of the membrane oxygen concentrations xi at zi, i = 2,...,n-1. This makes it possible to analyze the manner in which the facilitated transport depends on the parameters. For a physiological range of parameter values, the facilitated transport was found to increase as either the oxygen-myoglobin association rate constant k', the dissociation rate constant k, the oxygen diffusion coefficient, or the oxymyoglobin diffusion coefficient increases. Thus, the facilitated transport does not depend directly on ratios of chemical and diffusion coefficients. Although the increase in the oxygen diffusion coefficient does not per se affect the chemical conductance, it diminishes the membrane resistance through an interface feature, with a resulting increase in the facilitated transport. For a larger range of values of k' and k, the dependences of the facilitated transport on k' and on k are both biphasic and are similar in shape. However, the mechanisms involved are different: The associated changes in E(x1)-E(xn) and in gamma that result from the increase in k' are opposite to those that result from an increase in k. The use of (i) and (ii) permits, also, discrimination between the different roles of the physicochemical entities involved in a given facilitated transport change. In some cases (e.g., the decreasing phase of the facilitated transport as k' increases), this change depends in an essential manner on a secondary modification of the profile xi, i = 1,...,n, along the transport path.

Transfers between free and combined oxygen flows in determining facilitated transport with membranes on the transport path.

The facilitated transport of a species brought about by a diffusible carrier is mediated by the sharing of the total flow between the flow of the free species (fd) and of the species-carrier compound (fc). The presence of membranes impermeable to the carrier on the transport path is accompanied by transfers between fd and fc in regions next to the membranes. These transfers are associated with differences between the current species-carrier concentration and the one that would exist at chemical equilibrium with the free species. These differences determine that the actual facilitated transport is smaller than the possible maximum. Analytic approximations to describe these differences are obtained; they are expressed in terms of physiochemical entities germane to the transport. This leads to a better conceptual understanding of the process. A relatively small system of algebraic equations for the numerical solution of the facilitated transport is formulated. A feature of this approach is that, instead of matching the values of functions at some points, the flows derived from integrals evaluated over some intervals are matched. The expressions for the physicochemical entities obtained contain the parameters explicitly; this permits analysis of the mechanisms involved in the dependence of the facilitated transport on the parameters of the system. It is shown, for instance, that the increase of the "on" chemical coefficient may result in either an increase or a decrease of the facilitated transport, depending on the current values of the parameters.

Comparative facilitated transport of oxygen.

Numerical solutions were obtained for a model of facilitated transport of O2. The dependence of the facilitated flow on the concentrations of O2 at the two boundaries of the transport path was studied. The numerical values of the parameters correspond to the adult Ascaris lumbricoides and to vertebrate red striated muscle. A global control principle is formulated. This states that for every fixed O2 concentration at the low concentration boundary there exists an O2 concentration at the high concentration boundary for which the facilitated flow is maximum. The collection of these maxima makes possible the existence of a global adjustment of the facilitated transport in contradistinction to the mere presence of a local maximum. The ranges of the pairs of boundary O2 concentrations thus defined and for which the facilitated flow is within 70% of its attainable maximum were found to coincide with the physiological ranges of boundary O2 concentrations for the Ascaris and vertebrate striated muscle. This phenomenon has the character of a graded compensatory mechanism to hypoxia. It is an intrinsic property of the carrier transport system and does not depend on sensors for hypoxia.

Facilitated transport of oxygen in the presence of membranes in the diffusion path.

Most of the experimental observations on facilitated transport have been done with millipore filters, and all the theoretical studies have assumed homogeneous spatial properties. In striated muscle there exist membranes that may impede the diffusion of the carrier myoglobin. In this paper a theoretical study is undertaken to analyze the transport in the presence of membranes in the diffusion path. For the numerical computations physiologically relevant values of the parameters were chosen. The numerical results indicate that the presence of membranes tends to decrease the facilitation. For the nonlinear chemical kinetics of the reaction of oxygen with the carrier, this decrement also depends on the location of the membranes. At the higher oxygen concentration side of each membrane the flow of combined oxygen is transferred to the flow of dissolved oxygen. The reverse process occurs at the lower concentration side. Jump discontinuities of the concentration of the oxygen-carrier compound at each membrane are associated with these transfers. The decrement of facilitation is due to the cumulative effect of these jump discontinuities.